The great Arctic methane scare, again

A commentary posted last month in Nature: “Vast costs of Arctic change,” claims “A 50-gigatonne (Gt) reservoir of methane, stored in the form of hydrates, exists on the East Siberian Arctic Shelf. It is likely to be emitted as the seabed warms, either steadily over 50 years or suddenly.” The authors also claim such an event would cost us $60 trillion.  Here, I will address only the probability of a rapid release due to gradual warming.

This scare is not new.  Here is a headline from a 2007 methane scare article in The Canadian: “Over 4.5 Billion people could die from Global Warming-related causes by 2012.”

Methane hydrates occur in marine sediments and crop out on the ocean floor where the pressure is sufficiently high and the temperature is sufficiently low. (See Methane hydrates could fuel the world)

Methane  has been percolating from marine sediments for hundreds of years, at least, and has not suddenly appeared due to global warming.  Methane, as a greenhouse gas, is much stronger than carbon dioxide, but weaker than water vapor.

As geophysicist Judith Curry notes on her blog:

Most scientists who have specific knowledge in the area say a rapid release of methane due to warming is highly unlikely, even NASA’s Gavin Schmidt, a proponent of AGW and proprietor of RealClimate thinks the chances are very low.

Some other comments from Curry’s post:

“Permafrost hundreds of meters thick simply doesn’t warm or thaw much in ten years on account of its thermal inertia.”

“It’s not a given all the methane will end up in the atmosphere. Some could be oxidized [broken down] in the water by bacteria, and some could remain in the sediments on the sea floor.”

“…even if the ocean warms, most of the methane released by thawing permafrost could stay in the seabed or dissolve in seawater.”

An earlier paper in Nature, by  Carolyn D. Ruppel, “Methane Hydrates and Contemporary Climate Change” gives a good look at methane hydrate conditions.

“Catastrophic, widespread dissociation of methane gas hydrates will not be triggered by continued climate warming at contemporary rates (0.2ºC per decade; IPCC 2007) over time scales of a few hundred years. Most of Earth’s gas hydrates occur at low saturations and in sediments at such great depths below the sea floor or onshore permafrost that they will barely be affected by warming over even 1000 yr. Even when CH4 is liberated from gas hydrates, oxidative and physical processes may greatly reduce the amount that reaches the atmosphere as CH4.”

“Even when gas hydrate dissociates, several factors mitigate the impact of the liberated CH4 on the sediment-ocean-atmosphere system. In marine sediments, the released CH4 may dissolve in local pore waters, remain trapped as gas, or rise toward the sea floor as bubbles. Up to 90% or more of the CH4 that reaches the sulfate reduction zone (SRZ) in the near-sea floor sediments may be consumed by anaerobic CH4 oxidation. At the highest flux sites (seeps), the SRZ may vanish, allowing CH4 to be injected directly into the water column or, in some cases, partially consumed by aerobic microbes.

Methane emitted at the sea floor only rarely survives the trip through the water column to reach the atmosphere. At sea floor depths greater than ~100 m, O2 and N2 dissolved in ocean water almost completely replace CH4 in rising bubbles. Within the water column, oxidation by aerobic microbes is an important sink for dissolved CH4 over some depth ranges and at some locations. These oxidizing microbial communities are remarkably responsive to environmental changes, including variations in CH4 concentrations. For example, rapid deepwater injection of large volumes of CH4 led to dramatically increased oxidation in the northern Gulf of Mexico in 2010.”

Warming is unlikely to produce a sudden great release of methane.  A tectonic event, however, could possibly release large quantities of methane as has been postulated for the Paleocene-Eocene Thermal Maximum 55 million years ago. (See: Geologic History: PETM when it really got hot)

The graphic below, from Ruppel, shows the “habitat” for methane hydrates: